A spark plug for igniting a gas mixture in a combustion chamber is included in an internal combustion engine. In the spark plug, spark discharge occurs between electrodes spaced apart from each other, thereby igniting the gas mixture.
Various shapes and positions of the electrodes of the spark plug have been proposed before. Japanese Unexamined Patent Application Publication No. 2002-324650 will be referred to as patent document 1. For example, a spark plug described in patent document 1 is provided with a center electrode having a center electrode chip and a ground electrode having a ground electrode chip. The center electrode is disposed inside of a mounting fitting, and the ground electrode is disposed on a tip end of the mounting fitting. In the spark plug, the spark discharge occurs between a tip end face of the center electrode chip and a tip end face of the ground electrode chip.
In the spark plug, a part of the ground electrode having the ground electrode chip is non-perpendicular to a center axis of the mounting fitting. Thus, a direction from the ground electrode chip to the center electrode chip is non-perpendicular to a center axis of the center electrode. In addition, the ground electrode chip extends from a side face of the ground electrode to the center axis of the center electrode.
In such a configuration, a wider space between the ground electrode and the center electrode is secured in comparison to a configuration where the ground electrode extends to a position above the center electrode (i.e. position where the center axis of the center electrode and the center axis of the ground electrode are overlapped). Therefore, it is possible to prevent a phenomenon in which flame kernel occurring at a vicinity of the center electrode contacts with the surface of the ground electrode and in which a growth of the flame kernel is inhibited from occurring, and it is possible to exhibit excellent ignition performance. In addition, in the above-described configuration, it is also possible to obtain an effect that heat dissipation properties of the ground electrode become sufficient because the ground electrode is shortened.
In patent document 1, the center electrode chip mounted on the tip end of the center electrode is disposed non-perpendicular to the center axis of the center electrode. Thereby, the tip end face of the center electrode chip and the tip end face of the ground electrode chip face each other in a mutually parallel state. In such a configuration, even when a part of the center electrode chip is consumed with an occurrence of the spark discharge, a distance between the center electrode chip and the ground electrode chip, that is a discharge distance, may be constant. As a result, ignition performance of the spark plug may be stably maintained over a long period.
Like the spark plug described in patent document 1, in the spark plug having a configuration in which the ground electrode is non-perpendicular to the center axis of the mounting fitting, there is a problem that chips are difficult to be welded to respective electrodes when the tip end faces of the chips face each other in a mutually parallel state. This is because an installation and a condition needed to weld the chips to the electrodes are different from conventional ones when a center axis of each of the chips is non-perpendicular to a center axis of each of the electrodes which is an object of bonding. It is preferable that the center axis of the chip in a bonding part is perpendicular to a bonding face of the electrode for easily welding the chip to the electrode.